HCCI is a hybrid of two standard internal combustion processes: a spark-ignition (SI) process and a Diesel process. In an HCCI combustion process, typically the same fuel (gasoline) is used as in an SI process, but in contrast to standard SI, a spark plug is not used for ignition of the fuel. With respect to ignition, HCCI is more similar to a Diesel process in that the charge (fuel/air mixture) is compression ignited (i.e., auto-ignited).
Like Diesel fuel, gasoline is also auto-ignitable when heated to a sufficient high temperature. The typical auto-ignition temperature of gasoline is higher than for Diesel fuel, and is also higher than the typical temperature achieved in a cylinder at the end of the compression stroke in a typical four-stroke SI engine.
As discussed, for example, in the article “Use of Dynamic Valving to Achieve Residual-Affected Combustion”, by N. B. Kaahaaina et al., published as SAE Technical Paper Series No. 2001-01-0529 (2001), the HCCI engine operates in a four-stroke cycle, like the SI engine. The auto-ignition of the gasoline/air mixture at the end of the compression stroke is achieved by providing an elevated starting temperature at the beginning of the stroke in the HCCI engine in comparison to the temperature used in an SI engine. This elevated starting temperature is primarily achieved by either or both of two processes: a pre-heating of the air/fuel charge, or a reintroduction of some of the hot exhaust gasses from the previous cycle into the current cycle. In terms of efficiency, the latter method is typically superior.
After auto-ignition and during the combustion process, the typical HCCI engine achieves a higher average temperature in the cylinder, but a lower peak temperature as compared to SI. Furthermore, combustion sites are distributed throughout the cylinder, and there is generally no coherent flame front in the cylinder as occurs in SI engines. These attributes of HCCI combustion combine to yield a higher efficiency and a dramatic decrease in the amounts of certain types of pollutants.
However, the HCCI method of combustion also introduces certain disadvantages. A primary disadvantage is that HCCI is generally more suitable to low and medium load regimes than for higher load regimes. This stems primarily from the fact that HCCI uses a diluted charge, and hence cannot achieve the high peak power attributes capable when more concentrated charges are applied.
Two-stroke HCCI engines have been used, but these engines have been restricted to small, low-power applications which are not suitable, due to the typical drawbacks of the purely two-stroke cycle, for use in commercial vehicles. A first significant disadvantage of using a purely two-stroke engine in a commercial vehicle is that lubrication of two-stroke engines is a difficult problem. Lubrication oil often needs to be mixed in with the gasoline fuel, and therefore partially combusts in the cylinder directly introducing a large number of pollutants. A second significant disadvantage of purely two-stroke engines is that during the exhaust/intake cycle, there is a period of time when both the intake and exhaust ports are open, which allows a portion of the uncombusted fuel/oil mixture to flow out into the environment.
However, in a two-stroke cycle, the work-producing cycle happens twice as fast in comparison to a four-stroke cycle.
Accordingly, it would be advantageous to provide a system and method that achieves the advantages of the standard HCCI process while overcoming the inherent disadvantages and shortcomings of both purely four-stroke and purely two-stroke HCCI engines.